1. Field of the Invention
The present invention relates to optical lens systems, and, more particularly, to a four-member Gauss objective consisting of six lenses and having a bonded member with planar bond surfaces arranged on each side of the diaphragm, the objective having a high aperture ratio of at least 1:2.4 and a field angle which is greater than 40.degree..
2. Description of the Prior Art
Japanese Published Application Sho 46-10291 discloses an objective of this general construction. It has a relatively high aperture ratio and a large field angle.
From the U.S. Pat. No. 3,489,488 is known a Gauss objective which, in addition to having planar bond surfaces, also has a planar surface on the last lens in the direction of light travel, on the side facing toward the viewed object. However, this objective is limited to an aperture ratio of f:4 and a field angle of 30.degree.. Because this prior art objective has been designed with a view toward simplification of its construction and toward the possibility of using glass compositions of relatively low refractivity, lens designers have come to the conclusion that any cost-saving simplification of the above-mentioned basic construction of the Gauss objective could only be obtained in a trade-off in which a sacrifice in the imaging performance was made.
On the other hand, there exists the ever-present desire to maintain a certain performance level, once achieved with this type of Gauss objective, in spite of design changes which are aimed at the reduction of production costs.
The production costs of an objective are essentially determined by three cost components, viz. material costs, fabricating costs, and tooling costs. However, the indicated order of importance of the three major cost components applies only to large production quantities. Where quantities are smaller, the materials costs are least in importance, so that the design of these objectives must give first consideration to the fabricating time and to the tooling required, as well as to the mounting arrangement chosen.
A particularly economical mounting arrangement is disclosed in the German Offenlegungsschrift No. 2,364,621, the advantages of which are particularly significant in connection with the use of planar lens surfaces. Accordingly, the Gauss objective should have as many planar surfaces as possible, in order to maximize cost savings.
Using modern fabrication processes, the fabrication time can be reduced most effectively through the use of diamond tools. Tools of this type are very costly, however, and their use is economically justified only in connection with large production quantities. It follows that the aforementioned aim of providing as many planar surfaces as possible correspondingly increases the usefulness of these expensive tools, as fewer of them are required. By the same token, it is desirable that as many lens surfaces as possible have the same radius of curvature, so that fewer special tools are required and those that are required can be put to more efficient usage, thereby reducing fabrication costs accordingly.
These purely production-oriented considerations lead to a Gauss objective with lenses which have only planar-convex or planar-concave surfaces and which are arranged symmetrically in relation to the diaphragm. These requirements, however, signify that, from the very beginning, the degrees of freedom in terms of optical design parameters for correction calculations are considerably reduced. Adding to these constraints is the known characteristic of the above-mentioned Gauss objectives that the majority of lens distances and lens thicknesses can only be varied within a very small range. The only important correction approach remaining under these circumstances is the approach which involves the selection of the lens material for its optical characteristics.